How Would I Join The Selected Edges Together
I am not aware of an automagic way to do this, but here is a workaround: In edit mode, select one edge segment from the lower piece and the corresponding edge segment from the upper piece. Use [F] to fill a face between them. Continue doing this around the model. Because of the difference in vertex count, you will occasionally need to fill a triangle instead of a quad.

Here is some information on filling faces in edit mode:
https://www.blender.org/manual/modeling/meshes/editing/basics/creating_faces_and_edges.html

Edit: Filling faces is still useful, but Bridge is way better.

In edit mode, you'll select the edge loops, then Mesh > Edges > Bridge Edge Loops. Here's some information on edge tools—scroll down to the Bridge section: https://www.blender.org/manual/modeling/meshes/editing/edges.html